Dimming fluorescent lamps requires a minimum amount of output impedance to assure stable lamp operation at low light levels. It is known to provide this by using a resonant circuit in the output of the inverter, and modulating the duty cycle of the inverter waveform to regulate the light output of the lamp. This works well for linear fluorescent lamps, which have a relatively small value of negative incremental impedance and therefore a moderate increase in lamp impedance when their light output is reduced from full to low levels. In this context, lamp impedance is defined as the ratio of lamp arc voltage to arc current, while incremental impedance is the change ill arc voltage that results from a small change in arc current at a particular arc current. The presence of negative incremental impedance is characteristic of all fluorescent lamps, such that an increase in arc current causes a resulting decrease in arc voltage.
Compact fluorescent lamps, however, have a much greater negative incremental impedance characteristic and a much larger increase in lamp impedance as they are dimmed, so they require a correspondingly larger impedance from the resonant circuit to operate properly at low light levels. Therefore, when parallel-loaded resonant circuit components are sized for proper operation of compact lamps at low light levels, the lamp impedance at full light output is low enough that the circuit is so heavily damped as to no longer exhibit resonance effects. In essence, the resonant circuit then acts like a simple series choke ballast at full light output. This is not detrimental to the operation of the lamp, but it does provide an additional restriction that must be accounted for in the selection of the values used in the resonant circuit components. The inductor value can no longer be freely chosen, but must be designed to allow the proper full light output current to flow when the inverter is operating at its maximum output point, which corresponds to a duty cycle of 50%. With the inductor value fixed by the full output current requirements, the capacitor value is then also determined by the operating frequency, so that the resonant circuit impedance is fixed as well. However, it has been found that this impedance is not sufficient to allow stable operation of compact fluorescent lamps at low light levels in a ballast where only the duty cycle is varied to provide dimming control. In such a system, if one chooses resonant circuit values that operate the lamp properly at low end light levels, the ballast will be unable to deliver the current needed to allow the lamp to achieve full light output, and if the values are sized to allow full light output to be reached, the output impedance of the resonant circuit is insufficient to allow stable operation of the lamp at low light levels.
It is well known in the art to control the light output level of fluorescent lamps by changing the frequency of ballast operation, rather than the duty cycle. This can be done with either resonant or non-resonant ballast output circuitry, but it is most commonly achieved with resonant techniques. In one variation of this approach, the ballast has a series-loaded resonant output circuit which operates slightly above resonance when the lamp is at full light output and far above resonance when the lamp is at minimum light output. To dim the lamp, the frequency is shifted up above resonance and the series resonant circuit then acts much more like an inductor. This scheme is not suitable for compact fluorescent lamps or high performance dimming, because the lack of resonance at low light levels means that the output impedance is insufficient to allow stable lamp operation. It also can be problematic with regard to electromagnetic interference (EMI), since the wide variation of frequency needed to accomplish the dimming in this manner makes it difficult design a suitable EMI filter.
The use of parallel-loaded output circuits is also known in the ballast art. The assignee of the present application sells a fluorescent lamp ballast that incorporates a fixed frequency, variable duty cycle design, and another fluorescent lamp ballast that incorporates a variable frequency, fixed duty cycle design. Energy Savings Inc. of Schaumburg, Ill. and Advance Transformer of Chicago Ill. both have a fixed duty cycle, variable frequency fluorescent lamp ballast on the market. However, neither of these schemes is suitable for dimming compact fluorescent lamps. The fixed frequency, variable duty cycle design sold by the assignee of the present application has the problems detailed above, while the ESI ballast and the Advance Transformer ballast scheme suffer from the EMI difficulties inherent in any scheme that depends purely on frequency variation for dimming control.
The invention of the present application uses a parallel loaded resonant output circuit plus a combination of pulse width modulation and frequency variation to accomplish the dimming of compact fluorescent lamps. The invention implements a combination of variable duty cycle and variable frequency control, whereby the ballast operates at a fixed frequency throughout a selected range of light levels, with dimming control being done completely by duty cycle variation over this range of operation, and then smoothly moves to a variable frequency as the light output moves outside the selected range, with both duty cycle and frequency variation being the means of lamp light output control outside the selected range. Thus, for example, at high light levels, which are the most critical from the standpoint of EMI exposure, the ballast is essentially a fixed frequency unit and it is therefore relatively straightforward to design suitable EMI filtering as a result. As the lamp begins to approach the low light levels where output impedance becomes critical, the frequency is then shifted higher (towards resonance) and the required output impedance is thereby achieved. The additional degree of design freedom which the variation of frequency introduces allows the ballast designer to satisfy both the full lamp current criteria as well as the need for a proper output impedance at low light levels. One additional advantage of this technique is that the operation of the inverter switching devices can be maintained in the zero-voltage switching mode throughout the entire dimming range. With only duty cycle modulation, the switching devices do not operate in zero voltage switching mode at low light levels, which results in increased switching energy losses and additional heat and switching stress in the devices themselves.
In one embodiment, the invention encompasses an electronic dimming ballast for fluorescent lamps, arranged in use to supply to a fluorescent lamp an arc current from at least one controllably conductive device having a duty cycle and frequency of operation, the duty cycle and frequency of operation of the at least one controllably conductive device being independently controllable to adjust the light output of the lamp over a range of light outputs of the lamp from minimum to maximum.
The invention also encompasses an electronic dimming ballast for fluorescent lamps, comprising a circuit comprising at least one controllably conductive device for supplying a selected arc current to a fluorescent lamp to achieve a desired light output level from the lamp, a first circuit responsive to a dimming signal containing information representative of the desired light output level and generating an ac oscillator signal having a frequency determined by the dimming signal, and a second circuit responsive to the dimming signal for creating a duty cycle of operation for the at least one controllably conductive device at the frequency of the ac oscillator signal, the duty cycle being determined by the dimming signal, whereby the frequency and the duty cycle of operation of the at least one controllably conductive device are independently determinable over a range of desired light output levels of the lamp.
The invention also encompasses an electronic dimming ballast for fluorescent lamps, comprising an inverter circuit comprising at least one controllably conductive device for supplying a selected arc current to a fluorescent lamp to achieve a desired light output level from the lamp ranging from a minimum light output to a maximum light output, a first circuit for receiving a dimming signal containing information representative of a desired light level and generating a control signal representative of the desired light level, a second circuit responsive to the control signal for generating an ac oscillator signal having a frequency determined by the control signal, and a third circuit responsive to the control signal for creating a duty cycle of operation for the at least one controllably conductive device at the frequency of the ac oscillator signal, the duty cycle being determined by the control signal, whereby the frequency and the duty cycle of operation of the at least one controllably conductive device are independently determinable over the range of desired light levels from the minimum light output up to the maximum light output.
The invention also encompasses a method of selectably controlling the light output of a fluorescent lamp using an inverter circuit having at least one controllably conductive device for supplying a selected arc current to the fluorescent lamp to achieve a desired light output from the fluorescent lamp ranging from a minimum light output to a maximum light output, comprising the steps of generating a dimming signal variable from a state corresponding to a minimum light output of the lamp to a state corresponding to a maximum light output of the lamp, generating a control signal representative of the dimming signal, generating an ac oscillator signal having a frequency determined by the control signal, and generating a duty cycle of operation for the at least one controllably conductive device at the frequency of the ac oscillator signal, the duty cycle being determined by the control signal, whereby the frequency and the duty cycle of operation of the at least one controllably conductive device are independently determinable over the range of dimming signals variable from the state corresponding to the minimum light output up to the maximum light output.